1. Field of the Invention
The present invention relates to monolithic power amplifiers. More particularly, the present invention relates to a complementary metal oxide semiconductor (CMOS) Class AB power amplifier capable of output swings ranging from supply-to-supply. Other features of the design include: controlled DC bias current in the output devices and low distortion.
2. Description of the Prior Art
The development of integrated circuit technology has created unique problems when forming electronic circuits as monolithic structures. For example, linear CMOS circuits have been successful in the marketplace because of their low power consumption. However, problems unique to CMOS technology have presented a challenge to the design of CMOS linear circuits, that is, many common functional elements are not easily provided in a CMOS format or are unavailable altogether.
The limitations in prior art linear CMOS circuits have vexed manufacturers of integrated circuits for some time, and have prevented the production of an efficient, large dynamic range fully CMOS power amplifier. Prior art CMOS power amplifiers used output stage configurations that were subject to various limitations. FIG. 1 shows three of these prior art output stages: a Class A output stage (FIG. 1A), a Class B output stage (FIG. 1B), and a Class AB output stage (FIG. 1C).
The prior art Class A and Class AB output stages have a limited output voltage swing, and require large output driver devices in order to deliver the large amounts of current necessary to drive low resistive loads. The output voltage swing is limited by the threshold of the output driver devices and the maximum voltage the input of these output drivers can reach before the internal devices are driven out of saturation.
The prior art Class B output stage can drive large output voltage swings, however, it is subject to crossover distortion and uncontrollable DC bias current in the output driver devices. A configuration used to solve both of these problems has been provided in bipolar technology by including a diode circuit in series with the two inputs of devices 13a and 13b. CMOS technology is not amenable to such a solution due to the difficulty of forming such circuitry in CMOS devices having the necessary properties exhibited by bipolar devices.